And these two are also connected through radioactive decay.
So, here's the way it works, it's actually [UNKNOWN] tell you, super cool.
Hafnium is an element that is called a lithophile.
For all of you who know you Greek
out there, phile means friendly, litho is rock.
These are rock friendly elements.
That means, if it has a choice of where it's going to go, if it's, if the, if the
mantle were, were molten or, or things were freezing
out or something, hafnium would go into the rock part.
The other option is, something like Tungsten, Tungsten is siderophile.
Okay, we have the same file, it's friendly
to what, this is iron, it is iron friendly.
At some point in time, driven by all these
impacts, as the Earth was being formed, the Earth differentiated.
It was probably at first this big ball of
things that were iron and rock in many different
places strewn throughout, and as we know now, it
is iron on the inside and rock on the outside.
That process, as this process occurred, the hafnium would
stay here, and the tungsten, would go down here.
Most of the tungsten that was a, available in the
Earth should have sunk to the core, back when this happened.
An interesting thing happens, though.
Hafnium 182, has a half life, of 9 million years.
And when it decays, what does it decay into?
Tungsten 182.
Why is this good?
Let's think about this for a minute.
Let's think about all of this hafnium and this tungsten
in the originally all mixed up, hafnium is decaying into tungsten.
Finally differentiation occurs and it doesn't matter
where the tungsten started out with it
is siderophile, so when tungsten goes away leaving just a little bit of hafnium left.
Now we have all the tungsten in the core,
the hafnium is in the mantle now, and, what happens?
Well, whatever hafnium is left, a 182
still starts to decay, or continues to decay.
And suddenly, there is tungsten stranded in the mantel,
tungsten 182 is in the mantel when it shouldn't be.
Why does this help you?
Well, it gives you a time scale.
This 9 million timing scale is perfect for this.
It gives you time scale because if the differentiation
happens really fast, all of the tungsten goes down
really fast, then there's still a lot of hafnium
182 around, and it creates a lot of tungsten 182.
So if you find a lot of
tungsten 182, differentiation occurred really super early.
What if differentiation takes a long time, it takes a long
time for the planet finally to get to it's final state.
Well, by that time the hafnium is all decayed into tungsten 182.
The core happens and all the tungsten goes away and you look
in the mantle of the earth and you don't see any tungsten 182.
And what's the answer that you get when you do this?
Well, something like 30-100 million years for that last differentiation event.
Now, it's complicated, there're, there're many different
ways to interpret what this hafnium, tungsten
date really is telling you, but, I find it intriguing that this 3200 million
years is not dissimilar from those time scales that it takes dynamically for all
these, these planetary embryos to pull themselves
together, to finally make themselves, into a planet.
Those big impacts would have, have left
entirely, potentially entirely molten planet, and it would
be after those big impacts, when you
could finally differentiate and make this iron core.
When did that happen?
Well, maybe 30 to 100 million years, which is just what we expected.
Something really strange about hafnium tungsten though, if with respect to Mars.
We know the Mars hafnium-tungsten age
because we actually have meteorites from Mars.
We'll talk about, again, talk about meteorites in detail.
What do you get if you try to figure
out the hafnium tungsten age of the differentiation of Mars?
You get something like 5 million years.
That is a quite a bit shorter than these time scales here.
Basically, that means that the Martian meteorites are
filled with tungsten 182 because the differentiation happened
so fast, Hafnium was still all very active and all that 182 strapped in the mantle.
Why would these have a very different timescale?
[SOUND] That's a complicated question, and we will get to
it, in some of the very last lectures of this unit.
But it's it's a hard one to, to try to answer.
But, let's recap here what we know now.
We know now that terrestrial planets form the way
they do because they're being protected from the influence of
Jupiter by being just far enough away, and that they
form those oligarchs really fast, just like everyone else does.
But then they take of order 100 million years before
they really become the planets that we see around and love
today and that time scale is nicely matched with the hafnium
tungsten age of what we think these planets might really be.
Okay, now that we know where these terrestrial planets
came from and where the small bodies came from,
we can start to look at the bigger picture of how much of this stuff is really there.